Sequential writes to flash memory

ABSTRACT

This disclosure describes techniques for improving flash memory write performance using sequential writes. In some cases this is accomplished using a write buffer to store random writes. The stored random writes are written to the flash memory when sufficient random writes are stored to perform a sequential write.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/162,571 filed Mar. 23, 2009, the disclosure of which isincorporated by reference herein in its entirety. This application alsoclaims priority to U.S. Provisional Patent Application Ser. No.61/169,035 filed Apr. 14, 2009, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

In conventional NAND flash memory, flash cells are organized into pagesand pages are organized into blocks. Conventional NAND flash memory hassome design limitations, such as having to write an entire page of cellsto write a single flash cell. Further, if any of the flash cells in thepage have previously been written, those flash cells have to be erasedprior to writing. To erase those flash cells the entire block of pagescontaining those flash cells also has to be erased.

In part to address these design limitations, writing to flash cells ofNAND flash memory often involves copying data from every page of cellswithin a block to a buffer, erasing the entire block, modifying the dataassociated with the flash cells in the buffer, and writing every page ofcells within the block. This does not permit good data throughput inmany situations, especially those using random writes. A random write isa write of data to any randomly chosen location within the flash memoryand often involves no more than a few bytes of data.

SUMMARY

This summary is provided to introduce subject matter that is furtherdescribed below in the Detailed Description and Drawings. Accordingly,this Summary should not be considered to describe essential features norused to limit the scope of the claimed subject matter.

In one embodiment, a method is described that comprises storing firstdata associated with a first write request in a write buffer that isseparate from a flash memory, the first write request being configuredto request that at least part of the first data be written to the flashmemory, determining that the first data and second data associated witha second write request are sufficient for a sequential write to beperformed to the flash memory, and performing a sequential write of thefirst data and the second data to the flash memory.

In another embodiment, a method is described that comprises receiving arequest to write first data to a flash memory, determining that thefirst data is not sufficient to be written through a sequential write,storing the first data in a write buffer that is separate from the flashmemory until sufficient second data associated with one or moreadditional write requests is stored in the write buffer such that thesequential write is capable of being performed using the first and thesecond data, and performing the sequential write of the first and thesecond data to the flash memory in response to determining that thefirst and the second data are sufficient to perform the sequentialwrite.

In still another embodiment, a system is described that comprises aflash-memory controller, a write buffer separate from a flash memory,and a write-buffer controller configured to cause the flash-memorycontroller to store first data associated with a first write request inthe write buffer, the first write request configured to request that atleast part of the first data be written to the flash memory, determinethat the first data and second data associated with a second writerequest are sufficient for a sequential write to be performed to theflash memory, and perform a sequential write of the first data and thesecond data to the flash memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different instances in thedescription and the figures indicate similar or identical items.

FIG. 1 illustrates an operating environment in accordance with one ormore embodiments.

FIG. 2 illustrates a flash-controller system in accordance with one ormore embodiments.

FIG. 3 illustrates a method for improving flash-memory write performanceby storing write requests in accordance with one or more embodiments.

FIG. 4 illustrates a method for improving flash-memory write performanceby determining that a write request is not sufficient for a completesequential write in accordance with one or more embodiments.

DETAILED DESCRIPTION

This disclosure describes apparatuses and techniques for makingsequential writes to flash memory. In some cases this is accomplishedusing a write buffer to store random writes. The stored random writesare written to the flash memory when sufficient random writes are storedto perform a sequential write. Sequential writes are faster than randomwrites from a data throughput perspective. These techniques effectivelyincrease the data throughput of flash memory by grouping multiple randomwrites into one sequential write.

In the discussion that follows, an operating environment is described.Methods are also described that may be employed in the operatingenvironment as well as other environments. In the discussion below,reference will be made to the environment by way of example only and,therefore, implementations described below are not limited to theenvironment.

Operating Environment

FIG. 1 illustrates an operating environment 100 having aflash-controller system 102. Flash-controller system 102 iscommunicatively linked with computing device 104 through communicationlink 106 and with a flash memory 108 through communication link 110.Link 106 and link 110 is each a wired communication link and/or awireless communication link. Flash memory 108 may include multipleblocks of flash memory, such as NAND flash memory, and may be locatedwithin flash-controller system 102.

Flash-controller system 102 receives write requests 112 from computingdevice 104. Each write request 112 describes data to be written to flashmemory 108. After multiple write requests 112 have been received,flash-controller system 102 performs a sequential write 114 having datafrom these write requests 112. Sequential write 114 may be a partial orcomplete sequential write. A partial sequential write includes data thatfills at least two bytes worth of cells in a block (but less than all ofthe cells in a block) of flash memory 108. A complete sequential writeincludes data that fills all of the cells in a block of flash memory108.

FIG. 2 illustrates an example of a detailed embodiment offlash-controller system 102. Flash-controller system 102 includes awrite buffer 200 to which flash-controller system 102 stores writerequests 112. After sufficient write requests 112 have been stored toenable sequential write 114, flash-controller system 102 communicatessequential write 114 to flash memory 108 over communication link 110.

Flash-controller system 102 also includes flash-memory controller 202,system connector 204, storage connector(s) 206, backup power-source 208,backup flash-memory 210, and, in this embodiment, flash memory 108.Flash-memory controller 202 includes write-buffer controller 212 andpower-failure-backup handler 214. Write buffer 200 is acomputer-readable-medium such as dynamic random access memory (DRAM)with a capacity that is a substantial percentage of the total space offlash memory 108, such as twenty-five percent. Write-buffer controller212 stores write requests 112 and data associated with the writerequests 216 in write buffer 200. This data associated with the writerequests 216 is the data for which writing is being requested with thewrite requests 112.

Write-buffer controller 212 also stores metadata 218 in write buffer200. Metadata 218 includes information about data 216. Metadata 218 canbe used to track or organize data 216 using a data structure, such as aradix-tree data structure. Metadata 218 is updated to keep track ofblocks of data in flash memory 108 that have been updated by writerequests but which have not yet been written to flash memory 108. Whencomputing device 104 requests to read data, metadata 218 is checked tosee if the latest version (a most recently modified version) of the datais in write buffer 200. If the data is in a block that is in data 216,then the latest version of the data is in write buffer 200 (otherwise,the latest version of the data is not in write buffer 200). If thelatest version of the data is in write buffer 200, the data is read fromwrite buffer 200 instead of from flash memory 108. After data 216 iswritten to flash memory 108, metadata 218 is updated to remove entriesregarding the written data.

Metadata 218 may also include another radix-tree type of data structureto arrange multiple write requests 112 into one sequential write 114.Write-buffer controller 212 uses metadata 218 to translatelogical-block-addressing (LBA) to physical-block-addressing (PBA) andwrite changed blocks to flash memory 108 in a log-structured manner. Thesame radix-tree data structure can be used for this address translationto track data changes. By using the radix-tree to write back the data ina log-structured manner, wear leveling and associated garbage collectioncan be handled in write buffer 200, which is faster and allows flashmemory 108 to be available for writing data. Furthermore, flash memory108 may forgo space reserved for wear leveling and garbage collection.

Power-failure-backup handler 214 detects when power to flash-controllersystem 102 is lost, and in response to detecting that power is lostbacks up data 216 and metadata 218 to backup flash-memory 210. Backuppower-source 208 keeps flash-memory controller 202, backup flash-memory210, and write buffer 200 powered while handler 214 performs a backup ofdata 216 and metadata 216. Backup power-source 208 is an auxiliarybattery or a temporary power-source, such as super capacitors. Supercapacitors, also known as electric double-layer capacitors, areelectrochemical capacitors that have an unusually high energy densitycompared to that of common capacitors and usually operate at a lowerworking voltage. Backup flash-memory 210 is an area of flash memoryreserved for backup of write buffer 200. This reserved area of flashmemory is located within flash memory 108 or in a separate flash memorymounted on or external to flash-controller system 102.Power-failure-backup handler 214 also detects when power toflash-controller system 102 is subsequently restored, and in response todetecting that power has been restored restores data 216 and metadata218 to write buffer 200.

System connector 204 connects to communication link 106 (e.g., is aPeripheral Component Interconnect Express “PCIe” connector). Storageconnector 206 connects to communication link 110.

Note that one or more of the entities shown in FIGS. 1 and/or 2 may befurther divided, combined, and so on. Each of these entities can behardware, software, firmware, or a combination thereof, and/or stored oncomputer-readable-media and executed by one or more processors (notshown). For example, write-buffer controller 212 andpower-failure-backup handler 214 can be software and/or firmwareexecuted by flash-memory controller 202 or as driver software executedby computing device 104.

Also note that flash-controller system 102 can be implemented indifferent manners. Flash-controller system 102 can be a circuit boardincluding various integrated circuits (ICs), a system-on-chip (SoC), ora device including multiple circuit boards. Furthermore, flash memory108 can be included within flash-controller system 102 or as part of aseparate flash-memory device. Flash-controller system 102 can beincluded as part of various different devices, such as part of acomputing device, a portable flash-memory device, a video processingand/or rendering device, a television set-top box, or a gaming device.

Techniques for Sequential Writes

This disclosure describes techniques for improving flash memory writeperformance, such as by storing multiple writes into a sequential write.Two methods are illustrated below, aspects of which may be implementedin hardware, firmware, software, or a combination thereof. These methodsare shown as a set of acts that specify operations performed by one ormore entities and are not necessarily limited to the order shown.

FIG. 3 illustrates a method 300 for improving flash-memory writeperformance. At 302, data associated with a write request is stored in awrite buffer that is separate from a flash memory. Consider method 300in the context of FIGS. 1 and 2. At 302, write-buffer controller 212stores data 216 in write buffer 200. Write buffer 200 also containsadditional data from other write requests, such as from prior iterationsof operation 302.

At 304, the data and additional data associated with one or moreadditional write requests are determined to be sufficient for asequential write to be performed on the flash memory. Continuing theongoing example, at 304, write-buffer controller 212 determines, usingmetadata 218, that enough data 216 has been stored to enable asequential write 114 to be performed on flash memory 108. Dataassociated with two writes that affect cells in a same block but not allcells in the block is sufficient to perform a partial sequential write,which enables better performance than performing the writes separately.Data associated with two or more writes that affect all cells in a blockis sufficient to perform a complete sequential write.

At 306, a sequential write of the first data and the second data isperformed on the flash memory. At 306, write-buffer controller 212performs a sequential write 114 of at least a portion of data 216 toflash memory 108. This portion of data 216 is then removed from writebuffer 200 and metadata 218 is updated. The sequential write may be apartial or complete sequential write.

In some embodiments, the sequential write at 306 is performed inresponse to there being sufficient data for a sequential write (for apartial sequential or for a complete sequential write). In otherembodiments, the sequential write at 306 is performed more or less oftenand based on other factors. For example, write-buffer controller 212 maybe configured to determine when write buffer 200 is full or nearly full(e.g., in danger of becoming full such as by at least a threshold amountof buffer 200 being filled). In response to this determinationwrite-buffer controller 212 performs a sequential write. A block of datain flash memory 108 may have pending write requests 112 stored withinwrite buffer 200, for instance. While these pending write requests donot contain enough data to fill an entire block of cells (a completesequential write), it is still more efficient to write them sequentiallythan separately. Write buffer controller 212 combines these writerequests 112 into a partial sequential write request. This partialsequential write writes to a block of cells but does not contain datasufficient to write to every cell in the block. The corresponding areaof write buffer 200 is freed up for further use. If no combination ofwrite requests is found, a single write request may be performed.

Write-buffer controller 212 may use metadata 218 to select whichportions of data 216 to be written through partial sequential writes.For example, a block of data in flash memory 108 having more pendingwrite requests 112 stored within write buffer 200 is selected for apartial sequential write before a block of data in flash memory 108having fewer pending write requests 112 stored within write buffer 200is selected for a partial sequential write.

Further, write buffer controller 212 is configured, in some embodiments,to monitor the rate at which data is incoming via write requests 112. Ifthe incoming data rate is low, write buffer controller 212 performscomplete or partial sequential writes or single writes because they donot impact (or impact very little) overall data throughput. During thisperiod of low incoming data rate, write buffer controller 212 may alsopre-erase blocks of data in flash memory 108 for later use.

By storing random-write requests in write buffer 200, flash-memorycontroller 202 becomes immediately available to handle the next incomingwrite requests. This allows the incoming write requests to continue at ahigher rate than if flash-memory controller 202 waits for each writerequest to be performed prior to handling the next request. This in turnimproves overall data throughput.

Furthermore, conventional NAND flash memory employs wear levelingtechniques that shift data being written to blocks with lower cyclecounts (the number of times the block of cells has been written). Forrandom writes this wear-leveling leaves behind invalid data that islater cleaned up through a process known as garbage collection. Garbagecollection copies valid data from a block and then erases the blockfreeing it for later use. If the NAND flash memory is performing garbagecollection it is not immediately available for responding to writerequests. This results in additional write performance degradation. Wearleveling also oftentimes requires a particular amount (e.g.,approximately twenty-five percent) of the storage space of the flashmemory to be reserved for garbage collection purposes.

In some embodiments, write buffer controller 212 is configured totranslate logical-block-addressing (LBA) to physical-block-addressing(PBA) and write changed blocks to flash memory 108 in a log-structuredmanner. This allows write buffer controller 212 to maintain a record ofhow often blocks have been written (explicitly or relative to otherbocks in flash memory 108). By writing back the data in a log-structuredmanner, wear leveling and associated garbage collection can be handledin write buffer 200, which is both faster and allows flash memory 108 tobe available for writing data. For example, based on this record, writebuffer controller 212 can determine which blocks of flash memory 108 towrite to and/or for which blocks of flash memory 108 garbage collectionis to be performed. Furthermore, flash memory 108 may forgo spacereserved for wear leveling and garbage collection so that a particularamount of the storage space of the flash memory need not be reserved forgarbage collection purposes.

FIG. 4 illustrates an example of a method 400 for improving flash-memorywrite performance. At 402, a request to write data to a flash memory isreceived. In the context of FIGS. 1 and 2, write-buffer controller 212receives a write request 112 to write data to flash memory 108.

At 404, it is determined that the data is not sufficient to be writtenthrough a complete sequential write, such as by write-buffer controller212. The data requested to be written may not be sufficient to fill anentire block of cells in flash memory 108.

At 406, the data is stored in a write buffer that is separate from theflash memory. The data is stored until sufficient additional dataassociated with one or more other write requests is stored in the writebuffer such that a sequential write is capable of being performed usingthe data received at 402 and the additional data associated with one ormore other write requests. For example, at 406, write-buffer controller212 stores the data in write buffer 200 as part of data 216. Metadata218 is updated to account for this change to data 216.

At 408, it is determined that the data received at 402 and theadditional data associated with one or more other write requests aresufficient to perform a sequential write. This may be performed invarious manners as described above. Continuing the ongoing example,write-buffer controller 212 determines that the data received at 402 andthe additional data associated with one or more other write requests isstored in write buffer 200.

At 410, a sequential write of the data and additional data is performedon the flash memory. The sequential write is performed in response todetermining that sufficient data and additional data are stored in thewrite buffer such that a sequential write is capable of being performed.For example, at 410, once enough data 216 is stored sufficient to fill anew block of cells or to modify an existing block of cells, write-buffercontroller 212 performs sequential write 114 on flash memory 108. Thesequential write may be partial or complete.

In some embodiments, method 400 uses a partial sequential write ratherthan a complete sequential write. Operation 404, in this case, maydetermine that the data is not sufficient to be written until somethreshold percentage of an entire block of cells has been written to.This percentage can be modified depending on throughput requirements ora current rate of incoming write requests. If a current rate of incomingwrite requests goes up, the percentage increases. If the current rategoes down, the percentage decreases.

One or more of the techniques described above can be performed by one ormore programmable processors executing a computer program to performfunctions by operating on input data and generating output. Generally,the techniques can take the form of an entirely hardware embodiment, anentirely software embodiment, or an embodiment containing both hardwareand software components. In one implementation, the methods areimplemented in software, which includes but is not limited to firmware,resident software, microcode, etc. Furthermore, the methods can take theform of a computer program product accessible from a computer-usable orcomputer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system.

For the purposes of this description, a computer-usable orcomputer-readable medium can be any apparatus that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The medium can be an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system (or apparatus or device) or apropagation medium. Examples of a computer-readable medium include asemiconductor or solid state memory, magnetic tape, a removable computerdiskette, a random access memory (RAM), a read-only memory (ROM), arigid magnetic disk and an optical disk. Current examples of opticaldisks include compact disk-read only memory (CD-ROM), compactdisk-read/write (CD-R/W) and DVD.

Although the subject matter has been described in language specific tostructural features and/or methodological techniques and/or acts, it isto be understood that the subject matter defined in the appended claimsis not necessarily limited to the specific features, techniques, or actsdescribed above, including orders in which they are performed.

What is claimed is:
 1. A method comprising: storing first dataassociated with a first write request in a write buffer that is separatefrom a flash memory, the first write request being configured to requestthat at least part of the first data be written to the flash memory;detecting that a power failure has occurred; copying the first data fromthe write buffer to an area of a memory; detecting that power has beenrestored; restoring the first data from the area of the memory to thewrite buffer; determining that the first data and second data associatedwith a second write request are sufficient for a sequential write to beperformed to the flash memory; and performing a sequential write of thefirst data and the second data to the flash memory.
 2. The method asrecited in claim 1, wherein the sequential write is a completesequential write that includes data for every cell within a block of theflash memory.
 3. The method as recited in claim 1, wherein thesequential write is a partial sequential write and includes data for twoor more bytes worth of cells within a block of the flash memory.
 4. Themethod as recited in claim 1, wherein performing the sequential write isin response to determining that the write buffer is full.
 5. The methodas recited in claim 1, wherein performing the sequential write is inresponse to determining that a rate of incoming write requests is belowa threshold amount.
 6. The method as recited in claim 1, wherein thearea of the memory is an area of the flash memory.
 7. The method asrecited in claim 1, wherein the flash memory is a first flash memory andthe area of the memory is an area within a second flash memory that isseparate from the first flash memory.
 8. A method comprising: receivinga request to write first data to flash memory; determining that thefirst data is not sufficient to be written through a sequential write;storing the first data in a write buffer that is separate from the flashmemory until sufficient second data associated with one or moreadditional write requests is stored in the write buffer such that thesequential write is capable of being performed using the first and thesecond data; copying, prior to storing sufficient second data, the firstdata from the write buffer to an area of memory responsive to detectingthat a power failure has occurred; restoring the first data from thearea of memory to the write buffer responsive to detecting that powerhas been restored; and performing the sequential write of the first andthe second data to the flash memory in response to determining that thefirst and the second data are sufficient to perform the sequentialwrite.
 9. The method as recited in claim 8, further comprising:receiving a request to read data from the flash memory; determining thatthe data is within the write buffer; and communicating the data from thewrite buffer to satisfy the request to read the data.
 10. The method asrecited in claim 8, further comprising: performing, in a write buffercontroller that is separate from the flash memory, flash-memory wearleveling; and pre-erasing the block within the flash memory prior toperforming the sequential write on the block.
 11. A system comprising: aflash-memory controller; a write buffer separate from flash memory; anda write-buffer controller configured to cause the flash-memorycontroller to: store first data associated with a first write request inthe write buffer, the first write request configured to request that atleast part of the first data be written to the flash memory; detect thata power failure has occurred; copy the first data from the write bufferto an area of memory; detect that power has been restored; restore thefirst data from the area of memory to the write buffer; determine thatthe first data and second data associated with a second write requestare sufficient for a sequential write to be performed to the flashmemory; and perform a sequential write of the first data and the seconddata to the flash memory.
 12. The system as recited in claim 11, whereinthe system is implemented as a system-on-chip.
 13. The system as recitedin claim 11, further comprising: a Peripheral Component InterconnectExpress (PCIe) communication link to a computing device, wherein thefirst write request is received from the computing device via the PCIecommunication link.
 14. The system as recited in claim 11, furthercomprising a processor configured to communicate with the flash-memorycontroller, wherein the write-buffer controller is software executed bythe processor.
 15. The system as recited in claim 11, further comprisingthe flash memory.
 16. The system as recited in claim 11, wherein theflash memory is separate from the system and further comprising acommunication link to the flash memory.
 17. The system as recited inclaim 11, wherein the write buffer comprises dynamicrandom-access-memory (DRAM).
 18. The system as recited in claim 11,wherein the area of memory is within the flash memory.